Potential fix for code scanning alert no. 4: Workflow does not contain permissions

[C4illin]

Potential fix for https://github.com/C4illin/ConvertX/security/code-scanning/4

To fix this problem, add an explicit permissions block to restrict the default permissions of the workflow's GITHUB_TOKEN to the minimum necessary. In this case, since the following actions only require read access to repository contents (checking out the code and updating an external service), permissions: contents: read is sufficient. The best place is usually at the root (top-level of the workflow), immediately after the name: or before the env: block. This will enforce least-privilege access for the whole workflow and its jobs.

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Summary by cubic

Locks down the Docker Hub description workflow by adding permissions: contents: read to the GITHUB_TOKEN, resolving Code Scanning alert #4 and enforcing least privilege without changing behavior.

^{Written for commit f48d1aa. Summary will update on new commits.}